Wireless devices for communication such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two wireless devices, between a wireless device and a regular telephone and/or between a wireless device and a server, such as server providing video streaming service, via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, computers, or surf plates with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
A cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. eNodeB (eNB), NodeB, B node, Base Transceiver Station (BTS), or AP (Access Point), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Each cell may further comprise one or more antenna sites e.g. forming a combined cell or soft cell. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations also referred to as transmitter-receiver pairs. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to a wireless device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE is controlled by the radio base station.
License-Assisted Access via LTE (LAA-LTE) has recently been proposed as a technology for co-existence on unlicensed carriers with, e.g., WiFi. On an unlicensed carrier all data transmissions are to be controlled by a Listen-Before-Talk (LBT) mechanism, while control and management signaling may be sent without LBT. The LBT is a functionality which means that a channel such as an unlicensed channel is assessed to be clear before transmission begins.
Radio Link Control (RLC) Unacknowledged Mode (UM) is typically used for transmission of data between a node in a radio access network such as an eNB, and a UE, where the data packets are desired to be delivered in order and where RLC AM re-transmissions causes unacceptable long delays. UM is often used for time critical applications, for instance video streaming services, where delays due to RLC retransmissions cause larger degradation of the end user experience than packet loss. Therefore, current RLC UM protocol does neither support re-transmission nor re-segmentation.
Using an Unlicensed LTE carrier (ULC), for a time-critical and data packet order-critical service, UM Protocol Data Unit (PDU)s typically have to be produced before LBT is initiated. Thus if the ULC is occupied by other transmissions, some of the produced PDUs cannot be sent and will be discarded. Discarded PDUs can be re-produced to be transmitted at a later time, but these transmission re-attempts need to occur within a certain time budget from first attempt depending on the application's latency requirement Since UnLicensed Channel (ULC) access is unpredictable, some packets may be entirely discarded due to that ULC is occupied during the whole time budget for these packets. These discarded RLC UM PDUs will cause too high application packet loss rate causing significant degradation of user experience. For example, if RLC UM PDUs carries streaming video packets, then even a rather small amount of RLC UM packet loss can cause freezing video too and/or cause re-buffering.
It may be possible to use RLC AM protocol for time-critical application packet, but that would introduce RLC re-transmissions and larger RLC protocol overhead. RLC AM re-transmissions will not be beneficial for time-critical applications since a RLC AM re-transmission timers operate on a larger time-scale than the time budget and also that RLC AM cause additional overhead since receiver of the PDUs need to send status PDUs to sender.